An electron undergoes a transition from an initial (ni) to a final (nf ) energy state. The energies of the ni and nf energy states are −2.179×10−18J and −8.720×10−20 J, respectively.
Calculate the wavelength (λ) of the light in nanometers (nm) corresponding to the energy change (ΔE) value of this transition. You can use the following values for your calculations

Question

An electron undergoes a transition from an initial (ni) to a final (nf ) energy state. The energies of the ni and nf  energy states are −2.179×10−18J and −8.720×10−20 J, respectively.
Calculate the wavelength (λ) of the light in nanometers (nm) corresponding to the energy change (ΔE) value of this transition. You can use the following values for your calculations

abb0t · Answer

Given that you are already given energy, in joules, you can simply rearrange the equation for energy of a photon here, since energy transitions involve the loss of electrons.

So, tyou have: $\sf \color{red}{E=hv = h\frac{c}{\lambda}}$

E = energy
h = planks constant
v = frequency which is also given as speed of light over wavelength.

However, this transition, you can use the change in energy $\Delta$E = $\Delta$E$_f$-$\Delta$E$_i$ 

The rest is algebra, rearranging to solve for wavelength, $\lambda$

Can you do that - the algebra?